This application is based upon and claimed the benefit of priority of Japanese Patent Application Nos. 2001-132082 and 2002-088913 filed on Apr. 27, 2001 and Mar. 27, 2002, respectively, the contents of which are incorporated herein by reference.
(1) Field of the Invention
The present invention relates to a heater used as a heating source in office automation devices, household electric appliances, and precision machine, and more particularly to a fixing device for fixing a toner image in an office automation device such as a copying machine or a facsimile mounting the heater and an image forming apparatus using the fixing device.
(2) Description of the Description of the Related Art
To realize compact circuit boards, miniaturizing constituent components with multi-function have been promoted.
In an electronic copying machine, a toner image formed on a photosensitive drum is transferred onto a copy paper or copy film. The copy paper held between a heater and a pressure roller passes therethrough and is heated by the heater, thus the toner is fused and fixed on the paper.
This kind of heater is disclosed in U.S. Pat. Nos. 5,068,517, 5,162,635, and 5,587,097. These heaters are elongated thick heater elements formed on one surface of a heat-resistant and electrically insulating substrate composed of a ceramic material such as Al2O3 (aluminum oxide), AlN (aluminum nitride), or SiC (silicon carbide). The heater element contains Ag and Pd (silver and palladium) alloy and formed by conventional print-coating techniques.
A heater having the heater element with power supply terminals is known. The terminal composed of Ag (silver) or Pt (platinum) is coated on each one end of the heater element. The surface of the heater element is further covered with an insulating glassy overcoat layer or protective layer, thus the heater element is protected from wear and shock, increased in the mechanical strength, and protected from oxidation, sulfuration, and electric shock.
A heater, which provides a temperature detection element such as a thermistor, is also known. The temperature detection element, which is attached on the other surface of the substrate, detects temperature of the heater element and feeds back detection signals to a temperature control circuit device so that electric power to be applied to the heater is controlled and the temperature is kept constant.
A fixing device with such a heater has a good heat-up (temperature rise) characteristic and can heat toner almost directly, so that the thermal efficiency is high, and electric power consumption is saved, and further there is an advantage that the fixing device can be made compact.
As a material for forming the heater element of the heater, the aforementioned Agxe2x80x94Pd alloy is superior from the various viewpoints, and therefore it is widely used. In recent years, however, it is strongly desired to lower the amount of palladium in the alloy because palladium is apt to increase in cost of the heater.
A heater element for a heater disclosed in Japanese Patent publication No.H7-201459 is composed of an alloy of Ag (silver) and Pd (palladium) as its main component and the content of Pd (palladium) is relatively low (0.3 to 40 wt %). The patent publication also discloses that an active metal and a glass component of 10 wt % or less of the Agxe2x80x94Pd alloy are further contained in the heater element.
According to the description of the aforementioned patent publication, the heater element is formed on the substrate with high adhesive strength since the amount of palladium is reduced, and its heating and cooling speeds are fast and its temperature control can be made with high precision. However, when an active metal such as Ti or Zr is added, there is the possibility that it reacts with glass and may crystallize the heater element, which results in forming porous therein.
The heater requires the glass component as a binder for forming the heater element. However, the glass is apt to be degraded with time and by the heat cycle or due to the exposure of a high temperature. Therefore resistance of the heater element is apt to be varied with time.
In recent years, nitriding ceramics such as AlN (aluminum nitride, thermal conductivity 100 to 180 W/mk) or Si3N4 (silicon nitride) have been often used as substrates rather than substrates of Al2O3 (aluminum oxide, thermal conductivity about 20 W/mk) due to their superior thermal conductivities.
Substrate of nitriding ceramics, for example, AlN (aluminum nitride) contains nitrogen. When thick film patterns of a heater element, a power supply terminal, a wiring conductor and an overcoat layer are printed with paste on the AlN (aluminum nitride) substrate and is fired at about 850xc2x0 C., the AlN (aluminum nitride) substrate is reacted with the paste, and N2 gas (nitrogen) is generated from the substrate and bubbles of the N2 gas (nitrogen) is penetrated into the interface between the AlN (aluminum nitride) substrate and the patterns. Adhesion between the substrate and the patterns is reduced, which causes, for examples, peeling of the heater element.
In case of a substrate made of an oxide such as Al2O3 (aluminum oxide), generation of bubbles hardly occurs, and peeling is hardly caused because glass is composed of an oxide component.
In a heater element composed of an alloy of Ag (silver) and Pd (palladium) as its main component, when the content of Pd (palladium) is high, the temperature coefficient of resistance (TCR) of the heater element is reduced, and temperature of the heater element raises in a short time period after applying electricity thereto. However, a problem may arise when temperature rises excessively and a temperature control circuit does not operate.
A heater, which is mounted in a tray made of high-temperature synthetic resin, is equipped in a toner fixing device of a copying machine. The heater is generally used at 180 to 230xc2x0 C. for heating wax as one component of toner and for fixing toner images onto a paper. When temperature control of the heater cannot be performed properly by a failure of the temperature detection element or other reasons, power supply to the heater is not interrupted and an excessive current so flows into the heater that the heater element becomes red heat.
When the heater element is continuously red heat, the tray of synthetic resin mounting the heater, the frame of the fixing device, and a copy paper transferred toward the heater are burned. Other parts of the heater may also be affected by the red heat, such as carbonization by the red heat, or in an extreme case, it may cause smoke or fire though rarely.
The inventors studied on relations among the component ratio of an alloy of Ag (silver) and Pd (palladium) forming a heater element of a heater, the amount of a glass material functioning as binding and resistance adjustment means and the additive amount of an additive (a filler) other than the glass material, TCR characteristics and generation of bubbles.
The present invention was developed with the foregoing views and is intended to reduce the cost of a heater element formed on a substrate for a heater and to obtain the heater element having less variance in resistance among the heater elements manufactured and having the resistance falling within a predetermined ranges and minimize its fluctuations regardless of heat cycles by on and off power supply to the heat element and of use of the heater element under high temperature operational conditions. Furthermore, the present invention is intended to provide a heater preventing from the generation of bubbles between the substrate, wiring conductor, and overcoat layer and the glass and further preventing or reducing peeling of the heater element. The present invention is also intended to provide a fixing device using the heater, and an image forming apparatus with the fixing device.
The heater of the present invention has a substrate composed of a heat-resistant and electrically insulating material, a heater element formed on the surface of the substrate in a belt-shape in the longitudinal direction containing an alloy of Ag and Pd having a weight ratio Ag/Pd of 90/10 to 70/30, glass, and an inorganic oxide and/or an inorganic nitride of 0.1 to 20 wt % to the weight of the alloy of Ag and Pd, and a power supply terminal part formed in connection with the heater element.
The heater of the present invention may be a heater having a substrate composed of a heat-resistant and electrically insulating material, a heater element formed on the surface of the substrate in a belt-shape in the longitudinal direction containing an alloy of Ag and Pd having a weight ratio Ag/Pd of 90/10 to 70/30, a glass member and an inorganic oxide and/or an inorganic nitride as additive members, wherein a weight ratio of said additives is 10 to 70% to the weight of said alloy of Ag and Pd, and a power supply terminal part formed in connection with said heater element.
When the weight ratio x of the glass and inorganic oxide and/or inorganic nitride to the alloy of Ag and Pd and the formed sheet resistance y satisfy the following formula, the heater element of the heater can obtain a wider sheet resistance range:
y=aebx
where a and b are respectively within the ranges of 0.01xe2x89xa6axe2x89xa60.03 and b=0.08 to 0.12.
The present invention can be applied to a fixing device having a pressure roller in press contact with a recording medium and a heating device arranged opposite to the pressure roller for fixing an image formed on the recording medium, wherein the aforementioned heater is used for the heating device including the aforementioned heater element is arranged in opposite to the pressure roller.
The present invention is applied to an image forming apparatus having the above-mentioned fixing device and an image transferring device for adhering toner to an electrostatic latent image so as to form a toner image and transferring the toner image onto a recording medium.